Power semiconductor devices such as power Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) devices have become ubiquitous in a wide range of applications, from industrial applications such as heavy machinery, to consumer applications such as Heating, Ventilation, & Air Conditioning (HVAC) systems, electrical distribution systems and automotive systems. Power semiconductor devices are useful in that they can replace mechanical switches and relays with a fully solid-state device having a small form factor. In some situations, however, power semiconductor devices may pose a safety hazard due to device failure. For example, a power MOSFET device may fail in a short-circuit condition due to dielectric breakdown or metallization shorts that results in a low ohmic state. In this low ohmic state, control over the gate of the MOSFET is lost for a prolonged duration resulting in localized resistive heating of the device, which may create a hot spot within the device. Over time, this heating may lead to an undesirable thermal ignition of the package of the MOSFET device, the printed circuit board (PCB) on which the device is mounted, and/or the system in which the device installed.
Thermal ignition of power semiconductor devices is particularly dangerous in systems in which safety is a primary concern, such as in an automobile. If localized heating in a power semiconductor causes thermal ignition, the package ignites, which, in turn, ignites the PCB on which the package is mounted. The burning PCB may then set the automobile on fire.
Detecting the onset of a thermal overload condition is challenging because the onset of thermal overload is not necessarily accompanied by an easily detectable high current condition. If local heating within a particular device is caused by current crowding in a small region of the devices semiconductor material, high temperatures may be achieved within the device without appreciable current. Controlling or stopping the thermal overload condition may also be difficult if the device fails in a short-circuit condition that leaves the device uncontrollable. Some systems, have addressed this issue by coupling reflowable a thermal protection device (RTP) in series with the gate of the power semiconductor device that is designed to create an open circuit when the temperature of the RTP device exceeds a critical temperature.